Exploiting Commodity Hard-Disk Geometry to Efficiently Preserve Data Consistency

In the last couple of years, hard-disk technology has experienced an unjustified progressive boost of the built-in cache size, affecting both the power consumption and the reliability of stored data. Large built-in caches offer limited benefits in terms of performance with respect to the smaller ones. Moreover, they need to be kept in write-through mode to preserve data in case of a power failure in mission-critical systems. This implies severe repercussions on the disk write performance, due to the role of the built-in cache itself, mainly acting as a write scheduler, rather than just a mere I/O buffer, as its ever increasing size would suggest. In this scenario, an exact hard-disk characterization can provide the upper layers enough information to compensate the performance loss produced by the write-through policy. File-systems and device-drivers can in fact obviate most of these issues via proper data layouts, depending on a detailed knowledge of the hard-disks geometry. This paper introduces the chunk skew layout, a novel data layout strategy targeted to improve the performance of commodity hard-disks in missioncritical systems. For this purpose we also analyze the differences in terms of geometry and performance in a batch of identical commodity harddisks, discovering an unexpected and more complex scenario where most of the assumptions made so far on hard-disk technology do not hold